Fix "passwd: unrecognized option '--delete'" under busybox

Use `passwd -d` instead.

This fixes VMs like boot2docker, where we end up getting this error:

    passwd: unrecognized option '--delete'
    BusyBox v1.21.1 (2013-12-27 19:35:36 UTC) multi-call binary.

    Usage: passwd [OPTIONS] [USER]

    Change USER's password (default: current user)

            -a ALG  Encryption method
            -d      Set password to ''
            -l      Lock (disable) account
            -u      Unlock (enable) account

Tested on Mac OS X using dvm (https://github.com/fnichol/dvm)
Merge pull request #6 from dlitz/fix-busybox-delete-unrecognized

Fix "passwd: unrecognized option '--delete'" under busybox